The Afghan policeman who killed six US soldiers this week had tea with them before gunning the Americans down, Nato said today.

Investigators determined that Monday's shooting occurred after the US soldiers, working with Afghan National Army officers, climbed a mountain in Nangarhar province for an artillery drill.

After customary tea was served by the Afghan border police unit manning the outpost, the coalition forces fired a shell as part of the drill. Immediately after impact, one of the Afghan policemen opened fire, killing all six Americans. Two other US soldiers returned fire and killed the policeman.